The present invention relates to a cooling technique for a gas turbine moving blade.
In a high-temperature gas turbine conventionally used in a combined plant or the like, cooling air at a relatively low temperature is run through passages in a blade to keep the blade temperature lower than that of a high-temperature gas, in order to protect the blade against heat from the gas. In this air-cooling system for the blade, the cooling air supplied from a blade root section is run through the cooling passages in the blade, and then discharged into the space (main turbine gas) outside the blade through holes in the outer edge of the blade.
Recently, a blade cooling system based on steam has been proposed as the alternative to the air-cooling system. According to this steam-cooling system, used cooling steam is recovered without being discharged, so that the thermal efficiency of the gas turbine can be expected to improve. In the combined plant, moreover, the recovered steam can be fed to a steam turbine to improve the efficiency of the whole plant.
In a moving blade of the gas turbine, the high-temperature gas directly influences its wing section and platform section. Therefore, these sections must be cooled entirely and uniformly. Usually, as a cooling passage for cooling these sections, a so-called serpentine passage may be provided in the moving blade in a manner such that its inner surface extends along the outer surface of the wing or platform section.
Since the wing section has a substantial thickness, in this case, the aforesaid serpentine passage can be relatively easily provided therein even if the wing section is formed by precision investment casting. In contrast with this, the platform section is so thin and wide that it is difficult and uneconomical to form the serpentine passage therein throughout the area by precision investment casting.
In order to increase the general efficiency of the combined plant, a pressure loss must be restrained in the process of cooling the gas turbine blade so that the recovered steam can be kept at the highest possible pressure as it is supplied to the steam turbine. If the cooling steam is run through the serpentine passage that extends throughout the interior of the platform section, however, the pressure loss becomes too high to ensure a substantial improvement of the efficiency.